A tapered leaders helps in turning the fly over. When you just have tippet material, you're going from a strong, relatively stiff fly line straight to a weak, limp tippet. The tippet doesn't transfer the energy from the fly line out to the fly as well.

If you're using a big fly, and heavy tippet, you can usually get away with it, but if you're trying to toss tiny flies on light tippet, you'll want a leader of some kind.